I'm afraid I do not know for sure, but I imagine that mana regenerates based on max mana. I imagine that you gain maximum mana with every level up, and thus you will regenerate more mana with each level up.

In case you weren't aware, in Diablo 2 mana regenerated at around .8% per second. If mana follows a similar model, you will regenerate a % of max mana per second. Hopefully it is higher than .8%, as I believe a blue post somewhere suggested that they intended for classes to be able to spam their starting spells.

It's possible that as your level increases, the % of mana you regenerate decreases, so that you will eventually become reliant on one of the mana regeneration passives or one of the orange runestones on your attack spells.